The carbonylation of organic halides in the presence of transition metal catalysts, and especially of palladium type catalysts is well known. Hitherto this has been used for the synthesis of esters, amides and ketones, as well as of carboxylic acid derivatives. It is known to prepare aldehydes, but this has been done hitherto under a high pressure of carbon monoxide and hydrogen, Yoshida et al, Bull Chem Soc. Japan, 1976 49, 1681 and Schoenberg & Heck, JACS 1974, 96, 7761. The requirement of pressures of the order of 1500 psi and use of high-pressure reaction vessels is a considerable drawback. The present invention overcomes to a large extent the above drawbacks and provides a simple process which can be carried out under convenient conditions of reaction.